The present invention relates to a cast coated printing sheet. More particularly, the present invention relates to a cast coated printing sheet providing high paper gloss and smoothness, and enhanced printed product attributes. The present invention further relates to a method of manufacturing such a cast coated printing sheet.
There are three conventional casting methods of producing cast coated paper. All three involve pressing a coated substrate against a highly polished heated surface and adhering the coated substrate against the polished surface. The term xe2x80x9ccasting,xe2x80x9d as used herein, is intended to mean the step in which a coated substrate is pressed and adhered against the casting surface, regardless of the coated substrate""s physical state. The three methods are wet casting, gel casting, and rewet casting. In each case, the surface of the cast coated substrate exhibits the same gloss and smoothness as the highly polished casting surface.
The wet casting method includes (a) applying a coating to one surface of a web substrate, (b) pressing the coated side of the substrate against a casting drum, i.e., a highly polished, smooth cylindrical roll or continuous belt used in the manufacture of web substrates, (c) simultaneously with the pressing step, drying the coated side of the substrate against the casting drum, and (d) stripping the substrate from the casting drum with the dried coating adhered to the substrate. Typically the coated side of the substrate is pressed against the casting drum by means of a press roll. Very low casting drum temperatures, e.g., generally less than about 95xc2x0 C., are required in the wet casting method to ensure that the adhesion of the coating to the casting drum and the cohesive strength of the coating layer are greater than the vapor pressure of the liquid at the surface of the casting drum.
The gel casting method includes (a) applying a coating layer to one surface of a web substrate, (b) gelling the coating layer, (c) casting the gelled coating layer against a heated casting drum, and (d) stripping the coated substrate from the casting drum with the cast coating adhered to the substrate. Typically, the gelling step may be accomplished by chemically gelling the coating, e.g., by modifying the pH of the wet coating by an acidic or alkaline solution, such as formic acid or calcium formate, or by thermally gelling the coating, e.g., by using convection, conduction or infrared heating. Higher casting surface temperatures may be used in the gel casting method, e.g., up to about 120xc2x0 C., because the gelled coat has higher cohesive strength than a wet coating.
The rewet casting method includes (a) applying a coating layer to a surface of a web substrate, (b) drying the coating layer, (c) plasticizing the dried coating layer with a rewetting liquid, (d) casting the plasticized coating layer against a casting drum, and (e) stripping the coated substrate from the casting drum with the cast coating adhered to the substrate. The rewetting liquid is typically applied to the coated substrate in the nip formed between the press roll and the casting drum.
In addition to the three conventional casting methods, variations and combinations of these methods are known, e.g., after the gelling step, the gelled side of the substrate is further plasticized with a rewetting liquid in the nip formed between the press roll and the casting drum.
In order to be suitable for use in the processes described above, a coating layer should generally exhibit (a) sufficient adherence to the casting drum during the casting step, (b) moldability to the casting drum, and (c) adequate releasability from the casting drum at the completion of the casting step. Sufficient and uniform adherence to the casting drum ensures high surface gloss and smoothness, which are product attributes demanded by purchasers of cast coated sheets. If the coated substrate does not adhere sufficiently to the casting drum, the vapor pressure at the casting drum surface exiting the nip will typically exceed the adhesion force of the coating layer to the surface, resulting in blistering. If moldability of the coating layer to the casting drum surface is inadequate, pits, i.e., depressions in the surface of the coating layer, will tend to result and the product attributes of the cast surface will be unacceptable. If the release from the casting drum is inadequate, the coated layer of the substrate will tend to stick to the casting drum. The problems caused by inadequate release can range from picking, i.e., localized delamination of the coating layer from the underlying substrate, resulting in nonuniform surface characteristics, to macro-scale lamination of the coated substrate to the casting drum, resulting in unstable manufacturing operations.
To ensure sufficient adherence to the casting drum during the casting step, coating compositions for cast coating typically include high levels of a binder, such as latex and/orcasein. High levels of these materials, especially high levels of latex, can negatively affect releasability from the casting drum, resulting in, e.g., picking, and can deleteriously affect other product attributes, such as ink set time. To counteract excessive adherence to the casting drum, release agents are generally employed. Conventional means of improving releasability include applying a release agent to the surface of the casting drum, and adding a release agent to the coating composition, the gelling liquid and/or the rewetting liquid. Excessive amounts of release agent, however, can affect the aesthetic appearance and the printability of the cast coated sheet.
The coating compositions and the final product attributes have heretofore been constrained by the conventional methods of manufacture. Final product attributes as well as desired performance characteristics during the printing process are often sacrificed to design coating compositions that perform well during the casting process.
The present invention provides cast coated printing sheets providing high paper gloss and smoothness, desirable printing performance characteristics, and enhanced printed product attributes. Preferred cast coated printing sheets of the invention provide excellent printed product attributes for offset, rotogravure and flexographic printing applications. The invention further provides methods for manufacturing such cast coated printing sheets.
In one aspect, the invention provides a coated sheet for use in a cast coating process, comprising a substrate, and, on at least a first surface of the substrate, a dried coating comprising a latex and a pigment, wherein the latex is present in a functionally effective amount, i.e., an amount of latex that is selected so that a dried, unbrushed layer of the coating will not adhere sufficiently to a casting surface to allow the dried, unbrushed layer to be cast, and wherein an exposed surface of the dried coating has been brushed.
In another aspect, the invention provides a cast coated printing sheet comprising a substrate, and, on at least a first surface of the substrate, a coating comprising a latex and a pigment, wherein the latex is present in a functionally effective amount, and wherein an exposed surface of the coating has been brushed prior to casting.
In another aspect, the invention provides a cast coated printing sheet comprising a substrate, and, on at least a first surface of the substrate, a coating comprising a latex and a pigment, wherein the latex is present in a functionally effective amount, and the cast coated printing sheet has a 20xc2x0 gloss of greater than about 73.
In another aspect, the invention provides a cast coated printing sheet comprising a substrate, and, on at least a first surface of the substrate, a coating comprising a latex and a pigment, wherein the latex is present in a functionally effective amount, and the cast coated printing sheet has a 20xc2x0 ink gloss of greater than about 60.
In another aspect, the invention provides a cast coated printing sheet comprising a substrate, and, on at least a first surface of the substrate, a coating comprising a latex and a pigment, wherein the latex is present in a functionally effective amount, and the cast coated printing sheet has a PPS 10 kg Soft of less than about 0.50.
In another aspect, the invention provides a cast coated printing sheet having a 20xc2x0 gloss of greater than about 90.
The terms xe2x80x9cbrushed,xe2x80x9d xe2x80x9cbrushing,xe2x80x9d and variations thereof, as used herein, are intended to encompass a mechanical rubbing action on the surface of a substrate, such as the burnishing effect achieved by a buffer, brusher, or a friction calender. The term xe2x80x9cbrushing agent,xe2x80x9d as used herein, is intended to encompass materials that enhance the brushability of the coating layer, such as starch, plastic pigment, and polyvinyl alcohol.
Preferred embodiments may include one or more of the following features. The substrate is paper. The casting surface is a casting drum. The pigment is selected from the group consisting of precipitated calcium carbonate, aluminum trihydrate, satin white, high brightness clays and mixtures thereof. The coating further comprises a brushing agent present in a functionally effective amount. The xe2x80x9cfunctionally effective amountxe2x80x9d of brushing agent, as used herein, is an amount of brushing agent that is selected so that damage to the dried coating layer during brushing is minimized, i.e., no appreciable dusting occurs during brushing. The coating composition comprises about 5 to 25 parts of latex and about 1 to 30 parts of brushing agent, more preferably about 10 to 20 parts latex and about 3 to 20 parts brushing agent. The term xe2x80x9cparts,xe2x80x9d as used herein, means parts on a dry solids basis, and, as is well known in the art, parts are based on 100 parts of pigment. The latex is selected from the group consisting of styrene butadiene, modified styrene butadiene, styrene acrylate and mixtures thereof. The brushing agent is selected from the group consisting of plastic pigment, starch, polyvinyl alcohol and mixtures thereof. The coating composition comprises about 0.3 to 5.0 parts of a release agent, preferably selected from the group consisting of calcium stearate, polyethylene emulsion, beef tallow and mixtures thereof. The cast coated printing sheet is suitable for offset printing applications.
In another aspect, the invention provides a method of forming a coated substrate for use in a casting process comprising:
a) providing a substrate having a coated surface that comprises a dried coating that would not adhere to a casting surface sufficiently to perform a casting operation on the coating; and
b) brushing the dried coating to a sufficient extent so that the coating will adhere to the casting surface sufficiently to perform the casting operation on the coating.
In another aspect, the invention provides a method of forming a cast coated printing sheet comprising:
a) providing a substrate having a coated surface comprising a dried coating that, as dried, would not adhere to a casting surface sufficiently to perform a casting operation on the coating;
b) brushing the dried coating to a sufficient extent so that the coating will adhere to the casting surface sufficiently to perform the casting operation on the coating; and
c) after brushing, casting the coated surface.
In another aspect, the invention provides a method of forming a cast coated printing sheet comprising:
a) providing a substrate having a coated surface comprising a dried coating that, as dried, would not adhere to a casting surface sufficiently to perform a casting operation on the coating;
b) brushing the dried coating, wherein the net specific brushing intensity applied to the dried coating is about 0.002 to 0.1 kW hr/m2; and
c) after brushing, casting the coated surface.
Preferred methods may include one or more of the following features. The casting step is achieved by plasticizing the brushed coating with a rewetting liquid and by pressing the plasticized coating against a casting surface. A calendering step is performed before or after the brushing step, more preferably before the brushing step. The brushing step is performed by a brusher. The net specific brushing intensity is about 0.005 to 0.07 kW hr/m2. The casting surface is a casting drum. Steps (a) through (c) are repeated on a second opposite side of the substrate to create a cast surface on the second side of the substrate. More preferably, step (a) is performed on the first side and then repeated on the second side, then steps (b) and (c) are performed on the first side and then repeated on the second side. Most preferably, the calendering step is performed on both sides of the substrate before the brushing step.
In another aspect, the invention provides a method of printing comprising:
a) providing a cast coated printing sheet comprising a substrate, and, on at least a first surface of the substrate, a coating comprising a latex and a pigment, wherein the latex is present in a functionally effective amount, and wherein an exposed surface of the coating has been brushed prior to casting; and
b) printing an image on the printing sheet utilizing a printing press.
The term xe2x80x9cimage,xe2x80x9d as used herein, is intended to encompass any textual or pictorial image. Preferably the printing press includes offset, rotogravure and flexographic printing presses.